Case C-29/10: Judgment of the Court (Grand Chamber) of 15 March 2011 (reference for a preliminary ruling from the Cour d’appel — Luxembourg) — Heiko Koelzsch v État du Grand-Duché de Luxembourg (Rome Convention on the law applicable to contractual obligations — Contract of employment — Choice made by the parties — Mandatory rules of the law applicable in the absence of choice — Determination of that law — Notion of the country in which the employee ‘habitually carries out his work’ — Employee carrying out his work in more than one Contracting State)